Controlling Prosthetic Limbs with Electrode Arrays

Coiled conduits: The microscopic channels in this polymer roll are the right size and shape for bundles of severed nerve cells to grow through them. The scaffold, augmented with electrodes, is intended to transmit electrical signals between an amputee’s nervous system and prosthetic limb.

To design prosthetic limbs with motor control and a sense of touch, researchers have been looking at ways to connect electrodes to nerve endings on the arm or leg and then to translate signals from those nerves into electrical instructions for moving the mechanical limb. However, severed nerve cells on an amputated limb can only grow if a structure is present to support them—much the way a trellis supports a growing vine. And they are notoriously fussy about the shape and size of that structure.

“Cells are like people: they like furniture to sit in that’s just the right size,” says David Martin, a biomedical engineer at the University of Delaware. “They’re looking for a channel that’s got the ‘Goldilocks’-length scale to it—how far apart the ridges are, how tall they are, how [wide] they are.”

Ravi Bellamkonda’s lab at Georgia Tech has designed a tubular support scaffold with tiny channels that fit snugly around bundles of nerve cells. The group recently tested the structure with dorsal root ganglion cells and presented the results at the Society for Biomaterials conference earlier this month.

The scaffold begins as a flat sheet with tiny grooves, similar to corrugated iron or cardboard. It is then rolled to form a porous cylinder with many tiny channels suited for healthy nerve-cell growth. The floors of the conduits double as electrodes, brushing up close to the nerve bundles and picking up nerve signals. “The thing that’s different is that the patterns can be much more precisely controlled, and the orientation of the nerve bundles is essentially perfect here,” says Martin. “It’s a nice model system, and the ability to control nerve growth is what’s really going to be valuable.”

The ultimate goal is to enable two-way communication between the prosthetic limb and the wearer. Eventually, this design could separate the two kinds of nerve cells within a bundle, so neural cues directing hand movement would travel along one channel and information about touch and temperature from the prosthetic limb would travel to the brain along another channel. “The ‘jellyroll’ should in principle allow [them] to select through those channels—that to me is where the real excitement is,” says Martin. “That’s news for the future, but you’ve got to be able to walk before you can run.”